When removing sulphur during combustion of liquid or solid fuel containing sulphur, large quantities of preferably limestone or dolomite are consumed, calcium being the substance utilized. An efficient way of removing sulphur is to allow combustion to take place in a fluidized bed consisting at least partially of lime or dolomite.
The cost of procuring absorbent material and disposing of consumed absorbent material substantially increases the costs involved in the production of heat and electricity. Disposal of the consumed material also entails environmental problems. It is thus important, both from the expense and the environmental aspects, that the absorbent material is thoroughly utilized, preferably to almost 100%.
In what is known as a bubbling, fluidized bed, the gas speed is generally between 0.5 and 2.0 m/sec, and the particulate material in the bed behaves more or less like a liquid. Particles smaller than a certain size accompany the combustion gases leaving the bed and they therefore remain in the bed for only a short while. The duration of stay is also affected to a certain extent by the shape of the particles. With a gas speed of 1 m/sec, particles smaller than about 0.3-0.5 mm are blown away from the bed so quickly that their absorption capacity cannot be satisfactorily utilized. This means that the fraction of absorbent crushed to a size smaller than 0.3-0.5 mm is poorly utilized since its duration of stay in the bed is so short.